Sparteine oxidation is part of a genetic polymorphism that affects the metabolism of many drugs and is under monogenic control. By examining the sparteine oxidation kinetics and the ratio of the dehydrogenated metabolites and through the use of the potent inhibitor, quinidine, two sites of metabolism were found for all 10 of the livers studied. The mean Km (N = 10) for the quinidine-sensitive enzyme is 73 +/- 46 (SD) microM and the mean Vmax is 4.51 +/- 4.16 nmol/mg microsomal protein/30 min, indicating a large interindividual variation. Because the polymorphic defect is due to at least three variants of a mRNA splicing error with consequent lack of enzyme formation [Gonzalez et al.: Nature 331, 442 (1988)], the variation that we observed in Km is most likely due to variation of allozymes from extensive metabolizer alleles. The low affinity enzyme also demonstrates a large interindividual variation, is not competitively inhibited by quinidine, and produces a higher ratio of 5-dehydrosparteine to 2-dehydrosparteine than the high affinity enzyme. This low affinity enzyme must be part of a separate enzyme system from that controlling the sparteine/debrisoquine polymorphism because of its different characteristics and the 100% frequency with which it is found in the livers. The two dehydrosparteine metabolites are thought to be formed by the spontaneous breakdown of a primary metabolite. The different ratio of these two dehydrosparteines, which was found at low and high substrate concentrations, suggests that the reactions producing the primary metabolite are different between the quinidine-sensitive and -insensitive enzymes.